There are three
comfortable positions for playing my bass while sitting down, as shown in
the photos below.

(click on each photo to
enlarge)

This is the most common playing position that I use. The bass
balances nicely on my right leg, and my right arm rests comfortably in the
small arc in the upper body which is designed for that purpose. It
is a much more comfortable place for the right arm than on the curve of a
standard body bass, especially during long sessions.

I
use this position when playing intently or learning difficult passages.
The bass is propped across both legs, raising the instrument to a higher
and more visible position. My legs fit nicely into the two smaller
arcs of the lower body. Notice that the inverse horn of the upper
body curls comfortably around my chest, rather than poking into the
breastbone as is the tendency of a traditional body style.

This is my favorite position when playing with a pick. I put the
bass on my left leg at roughly a 45 degree angle, and put the arc of the
heel of the bass against my right thigh.